The invention relates to techniques for reducing and/or correcting for borehole effects encountered in subsurface measurements. More particularly, the invention concerns methods, and devices for their implementation, to reduce or correct for undesired electromagnetic effects associated with the deployment of antennas having transverse or tilted axes within a borehole.
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically employ logging instruments or xe2x80x9csondesxe2x80x9d equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy interacts with the surrounding formation to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile of the formation properties is obtained.
Electromagnetic (EM) logging techniques known in the art include xe2x80x9cwirelinexe2x80x9d logging and logging-while-drilling (LWD). Wireline logging entails lowering the instrument into the borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD entails attaching the instrument disposed in a drill collar to a drilling assembly while a borehole is being drilled through earth formations.
Conventional wireline and LWD instruments are implemented with antennas that are operable as sources and/or sensors. In wireline applications, the antennas are typically enclosed by a housing constructed of a tough plastic material composed of a laminated fiberglass material impregnated with epoxy resin. In LWD applications, the antennas are generally mounted on a metallic support to withstand the hostile environment encountered during drilling. Conventional logging instruments are also being constructed of thermoplastic materials. The thermoplastic composite construction of these instruments provides a non-conductive structure for mounting the antennas. U.S. Pat. No. 6,084,052 (assigned to the present assignee) describes implementations of composite-based logging instruments for use in wireline and LWD applications.
In both wireline and LWD applications, the antennas are mounted on the support member and axially spaced from each other in the direction of the borehole. These antennas are generally coils of the cylindrical solenoid type and are comprised of one or more turns of insulated conductor wire that is wound around the support. U.S. Pat. Nos. 4,873,488 and 5,235,285 (both assigned to the present assignee), for example, describe instruments equipped with antennas disposed along a central metallic support. In operation, the transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (also referred to herein as mud) and into the formation. The signals detected at the receiver antenna are usually expressed as a complex number (phasor voltage) and reflect interaction with the mud and the formation.
One EM logging technique investigates subsurface formations by obtaining electrical resistivity or conductivity logs by xe2x80x9cfocusedxe2x80x9d measurements. U.S. Pat. No. 3,452,269 (assigned to the present assignee) describes an instrument adapted for taking these focused measurements. The technique described in the xe2x80x9c269 patent uses a survey current emitted by a principal survey current emitting electrode. This survey current is confined to a path substantially perpendicular to the borehole axis by focusing currents emitted from nearby focusing electrodes. U.S. Pat. No. 3,305,771 describes a focusing technique using an instrument equipped with toroidal coils. U.S. Pat. Nos. 3,772,589, 4,087,740, 4,286,217 (all assigned to the present assignee) describe other electrode-type instruments used for subsurface measurements.
U.S. Pat. No. 5,426,368 (assigned to the present assignee) describes a logging technique using an array of current electrodes disposed on a support. The xe2x80x9c368 patent uses the electrode configuration to investigate the geometrical characteristics of the borehole and the resistivity properties of the formation. U.S. Pat. Nos. 5,235,285 and 5,339,037 (both assigned to the present assignee) describe metallic instruments adapted with a toroidal coil and electrode system for obtaining resistivity measurements while drilling. The measurement techniques described in the xe2x80x9c285 and xe2x80x9c037 patents entail inducing a current that travels in a path including the conductive support body and the formation.
U.S. Pat. Nos. 3,388,325 and 3,329,889 (both assigned to the present assignee) describe instruments equipped with an electrode and coil configuration for obtaining subsurface measurements. U.S. Pat. No. 3,760,260 (assigned to the present assignee) also describes a downhole instrument equipped with electrodes and coils. The xe2x80x9c260 patent uses the electrode configuration to ensure radial current flow into the formation surrounding the borehole. U.S. Pat. No. 4,511,843 (assigned to the present assignee) describes a logging technique whereby currents are emitted from electrodes to zero a potential difference between other electrodes on the instrument. U.S. Pat. No. 4,538,109 (assigned to the present assignee) describes a logging technique aimed at correcting or canceling the effects of spurious EM components on downhole measurement signals.
A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area. The direction and strength of the magnetic moment can be represented by a vector perpendicular to the plane of the coil. In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the instrument. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole.
An emerging technique in the field of well logging is the use of instruments incorporating antennas having tilted or transverse coils, i.e., where the coilxe2x80x9ds axis is not parallel to the support axis. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). The aim of these TMD configurations is to provide EM measurements with directed sensitivity and sensitivity to the anisotropic resistivity properties of the formation. Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, 6,147,496, WO 00/50926, and in V. F. Mechetin et al., TEMPxe2x80x94A New Dual Electromagnetic and Laterolog Apparatusxe2x80x94Technological Complex, Thirteenth European Formation Evaluation Symposium Transactions, Budapest Chapter, paper K, 1990.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is much more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current along the borehole axis. When it is eccentered in a direction parallel to the direction of the magnetic moment, the symmetry of the situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the magnetic moment, axial currents are induced in the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD receivers, they can cause undesired signals that are many times larger than would appear in a homogeneous formation without a borehole.
U.S. Pat. No. 5,041,975 (assigned to the present assignee) describes a technique for processing signal data from downhole measurements in an effort to correct for borehole effects. U.S. Pat. No. 5,058,077 describes a technique for processing downhole sensor data in an effort to compensate for the effect of eccentric rotation on the sensor while drilling. However, neither of these patents relates to the properties or effects of TMDs in subsurface measurements.
Thus there remains a need for improved methods and apparatus for reducing or correcting for these currents when using well logging instruments implemented with TMDs.
The invention provides an apparatus for use in a borehole traversing a subsurface formation, comprising an elongated support having a longitudinal axis; at least one antenna disposed on the support such that its axis is tilted or perpendicular with respect to the support axis, each antenna adapted to transmit or receive electromagnetic energy; a first pair of electrodes disposed on the support near the at least one antenna; the first electrode pair adapted for joint electromagnetic interaction; and a second pair of electrodes disposed on the support and adapted to detect a current within the borehole; wherein the electromagnetic interaction between the first electrode pair is associated to the detection of a current at the second electrode pair.
The invention provides an apparatus for use in a borehole traversing a subsurface formation, comprising an elongated support having a longitudinal axis; at least one antenna disposed on the support such that its axis is tilted or perpendicular with respect to the support axis, each antenna adapted to transmit or receive electromagnetic energy; and a pair of electrodes disposed on the support, the electrodes linked to one another for joint electromagnetic interaction; wherein the electrodes of the electrode pair are adapted to selectively form a short or open circuit between the electrodes.
The invention provides a method for making a subsurface measurement. The method comprises transmitting electromagnetic energy from an antenna disposed within a subsurface borehole with its axis tilted or perpendicular with respect to the borehole axis; detecting a current flow along the borehole, the current flow associated with the electromagnetic energy transmission, with a first pair of electrodes disposed within the borehole; and emitting a current between a second pair of electrodes disposed within the borehole in response to the current flow detected at the first electrode pair to reduce the current flow; and determining a voltage signal associated with the transmitted energy.
The invention provides a method for making a subsurface measurement. The method comprises disposing a first pair of electrodes within the borehole, the electrodes adapted to emit a current therebetween; disposing an antenna within the borehole with its axis tilted or perpendicular with respect to the borehole axis, the antenna adapted to transmit electromagnetic energy; activating either antenna to transmit electromagnetic energy or the first electrode pair to emit a current between said electrodes; measuring a potential between a second pair of electrodes disposed within the borehole; activating either antenna or the electrode pair that was not activated above and respectively deactivating whichever of same was activated above; measuring a potential between the second electrode pair; and determining a voltage signal using the measured potentials.